The code does not compile:
Test.java:8: method() in Bar cannot
override method() in Foo; overridden
method is static final
public static void method() {
The message is misleading since a static method can, by definition, never be overridden.
I do the following when coding (not 100% all the time, but nothing here is "wrong":
(The first set of "rules" are done for most things - some special cases are covered after)
- create an interface
- create an abstract class that implements the interface
- create concrete classes that extend the abstract class
- create concrete classes that implements the interface but do not extend the abstract class
- always, if possible, make all variables/constants/parameters of the interface
Since an interface cannot have static methods you don't wind up with the issue. If you are going to make static methods in the abstract class or concrete classes they must be private, then there is no way to try to override them.
Special cases:
Utility classes (classes with all static methods):
- declare the class as final
- give it a private constructor to prevent accidental creation
If you want to have a static method in a concrete or abstract class that is not private you probably want to instead create a utility class instead.
Value classes (a class that is very specialized to essentially hold data, like java.awt.Point where it is pretty much holding x and y values):
- no need to create an interface
- no need to create an abstract class
- class should be final
- non-private static methods are OK, especially for construction as you may want to perform caching.
If you follow the above advice you will wind up with pretty flexible code that also has fairly clean separation of responsibilities.
An example value class is this Location class:
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public final class Location
implements Comparable<Location>
{
// should really use weak references here to help out with garbage collection
private static final Map<Integer, Map<Integer, Location>> locations;
private final int row;
private final int col;
static
{
locations = new HashMap<Integer, Map<Integer, Location>>();
}
private Location(final int r,
final int c)
{
if(r < 0)
{
throw new IllegalArgumentException("r must be >= 0, was: " + r);
}
if(c < 0)
{
throw new IllegalArgumentException("c must be >= 0, was: " + c);
}
row = r;
col = c;
}
public int getRow()
{
return (row);
}
public int getCol()
{
return (col);
}
// this ensures that only one location is created for each row/col pair... could not
// do that if the constructor was not private.
public static Location fromRowCol(final int row,
final int col)
{
Location location;
Map<Integer, Location> forRow;
if(row < 0)
{
throw new IllegalArgumentException("row must be >= 0, was: " + row);
}
if(col < 0)
{
throw new IllegalArgumentException("col must be >= 0, was: " + col);
}
forRow = locations.get(row);
if(forRow == null)
{
forRow = new HashMap<Integer, Location>(col);
locations.put(row, forRow);
}
location = forRow.get(col);
if(location == null)
{
location = new Location(row, col);
forRow.put(col, location);
}
return (location);
}
private static void ensureCapacity(final List<?> list,
final int size)
{
while(list.size() <= size)
{
list.add(null);
}
}
@Override
public int hashCode()
{
// should think up a better way to do this...
return (row * col);
}
@Override
public boolean equals(final Object obj)
{
final Location other;
if(obj == null)
{
return false;
}
if(getClass() != obj.getClass())
{
return false;
}
other = (Location)obj;
if(row != other.row)
{
return false;
}
if(col != other.col)
{
return false;
}
return true;
}
@Override
public String toString()
{
return ("[" + row + ", " + col + "]");
}
public int compareTo(final Location other)
{
final int val;
if(row == other.row)
{
val = col - other.col;
}
else
{
val = row - other.row;
}
return (val);
}
}
